Silver is said to be the most preferable material for a reflection film being used in an optical recording medium, display or the like. The reason is that silver, in addition to a high reflectivity, is less expensive than gold which also has a high reflectivity. Especially, in the field of optical recording media, application of materials having higher reflectivity has been demanded as the field makes the transition to recordable and rewritable media such as CD-R/RW and DVD-R/RW/RAM. This is ascribable to a cause that organic dye materials have been widely used as a constituent material for a recording layer in recordable and rewritable media, and organic dye materials lead to a greater attenuation of laser, so that the attenuation should be complemented by improving the reflectivity of a reflection film.
In contrast, silver is poor in corrosion resistance, thereby has a drawback that corrosion changes the color of silver into black and will deteriorate the reflectivity. One of the factors contributing a corrosion of a reflection film is that for example the film has a poor corrosion resistance to an organic dye material which is applied to a recording layer in an optical recording medium although it varies according to equipment on which the film is applied, and as a result deterioration in reflectivity can be recognized after a long-term use. A reflection film in a display is likely to be corroded due to moisture or the like in the atmosphere.
There is further an issue of deterioration in reflectivity to a reflection film comprising silver caused by heat. Although it is not known exactly how heat deteriorates a reflectivity, a phenomenon has been confirmed that when a silver thin film is heated, the thin film gets local coagulation and exposes its substrate layer. Therefore, heat resistance is also required to a reflection film used for an optical recording media, plasma display or the like because the film is subjected to heat.
In order to address the above-discussed issue of deterioration in a reflectivity of a reflection film, developments of a silver alloy for use in a reflection film have been conventionally carried out in which both corrosion resistance and heat resistance are increased while a reflectivity is insured. Many of the reflection films have silver as a main component and are added by one or two or more various additive elements. For instance, it is disclosed 0.5-4.9 atm % of palladium has been added to silver. These silver alloys are said to have a good corrosion resistance, can maintain reflectivity under an environment of service, thereby being suitable as a reflection film. Refer to Patent Document 1 for details of the earlier technology. Patent Document 1: Japanese Patent Application Laid-open No. 2000-109943 With respect to the above-discussed silver alloys, some tentative improvements are recognized in terms of corrosion resistance and heat resistance. However, there is no guarantee that these silver alloys do not deteriorate at all under an environment of service. There is also no guarantee against deterioration in reflectivity, and so that materials capable of maintaining a higher level of reflectivity have been demanded.
In the field of optical recorders, red semiconductor laser (wavelength of 650 nm) is currently applied as a light source for recording, however practical use of blue laser (wavelength of 405 nm) has been in sight recently. Application of blue laser can insure a five- or six-fold storage capacity compared to the present optical recorders, so that it is considered blue laser-applied optical recorders will become mainstream in the next-generation ones. In this regard, the present inventors have confirmed the reflectivity of a reflection film will fluctuate in accordance with the wavelength of laser to be irradiated, and especially have confirmed irradiation of laser having a short-wavelength will cause deterioration in reflectivity irrespective of presence or absence of corrosion and the deterioration in reflectivity due to corrosion will be more substantial in degree than in a case of irradiation of laser having a long-wavelength. Thus, in order to manufacture recording media capable of addressing the future development of light sources for recording, it is desired a development of materials having a higher reflectivity with respect to irradiation of laser of short-wavelength band and further capable of maintaining reflectivity within a practical scope of use.
The present invention has been made against the above background, and relates to a silver alloy composing a reflection film in an optical recording medium or the like. It is an objective of the invention to provide a material capable of serving with the reflectivity kept not deteriorated due to a long-term use. Furthermore, the present invention provides a material having a higher reflectivity with respect to laser beams of short-wavelength.